Easily poled 0-3 piezoelectric composites for transducer applications

ABSTRACT

A PET-polymer composite of 0-3 connectivity further includes a third phase particulate additive of conductive or semiconductive material to increase the electrical conductivity of the polymer matrix.

This application is a division of application Ser. No. 022,519, filedMar. 6, 1987 now U.S. Pat. No. 4,944,891, issued 7/31/90, which is acontinuation of application Ser. No. 742,333, filed June 7, 1985.

BACKGROUND OF THE INVENTION

This invention is related to piezoelectric materials and, moreparticularly, to a 0-3 PZT-polymer composite for transducer applicationsand the like.

Piezoelectricity can be explained as electric polarization produced bymechanical stress in certain substances. Piezoelectric materials, suchas lead zirconate titanate (PZT), are used in a wide variety ofapplications. In hydrophone devices piezoelectric materials detect lowfrequency acoustic waves passively. Hydrophones are frequently made fromsingle phase PZT. Large hydrostatic piezoelectric charge and voltagecoefficients (d_(h) and g_(h)) are desired in these devices. Thus, eventhough the magnitudes of the piezoelectric coefficients d₃₃ and d₃₁ ofPZT are large, the hydrostatic coefficients d_(h) and g_(h) are small,because the d₃₃ and 2d₃₁ coefficients are almost equal and opposite insign, and also, the dielectric constant of PZT is large. The largedifference of the acoustic impedance between PZT and water requiresimpedance matching layers for underwater hydrophone applications.

In order to improve and modify material properties for hydrophonedevices, several different types of piezoelectric PZT-polymer compositeshave been recently investigated utilizing the concept of phaseconnectivity. It has been found that the electric flux pattern and themechanical stress distribution together with the resulting physical andpiezoelectric properties depend strongly on the manner in which theindividual piezoelectric and polymer phases of the diphasic compositesare interconnected. Each phase in a composite may be self-connected inzero, one, two, or three dimensions. Thus, a diphasic 2-1 connectivitypattern, for example, has one phase self-connected in two dimensionallayers, the other in one dimensional chains or fibers. Below arerepresented some of the composite with different connectivity patternsin which the piezoelectric phase appears first.

0-3 composite: PZT particles suspended in a polymer matrix

1-3 composites; PZT rods aligned in the poling direction held togetherby a polymer matrix

1-3-0 composites: PZT rods aligned in the poling direction held togetherby a foamed polymer matrix

3-1 and 3-2 composites: holes drilled in a prepoled PZT block, then theholes filled or covered by polymer.

3-3 composites: lost-wax method using coral as the starting material, orby a fugitive phase method (BURPS process).

The d_(h), g_(h) coefficients and d_(h) g_(h) figure of merit of thediphasic composites are significantly improved over single phase PZT dueto decoupling of the d₃₃ and d₃₁ coefficients and/or the reduction ofthe dielectric constant.

The piezoelectric ceramic-polymer composites of 1-3, 1-3-0, 3-1, 3-2,and 3-3 connectivities are often expensive and cumbersome to fabricate.The PZT-polymer 0-3 composite is relatively easy and inexpensive tomake. However, in 0-3 composites, early studies showed that the PZTparticles should have a diameter greater than the thickness of thecomposites to obtain sufficient poling. For smaller particles of PZT,very large poling field strength (≃100-150 kV/cm) are needed to achievesufficient poling.

For a 0-3 composite consisting of spherical grains embedded in a matrix,the electric field E₁, acting on an isolated spherical grain is give by##EQU1## In this equation, K₁ and K₂ are the dielectric constants of thespherical piezoelectric grains and the polymer matrix, respectively, andE₀ is an externally applied electric field. For a 0-3 composite of PZTpowder and polymer, K₁ is about 2000 and ₂ about 5. In such a compositewith an external field of 100 kV/cm, the electric field acting on thepiezoelectric particles is only about 1 kV/cm which is insufficient topole the composite. According to the above equation E₁ ˜E₀ only when thedielectric constant of the piezoelectric phase approaches that of thepolymer phase. Most of the ferroelectric materials have very highdielectric constants and hence the above condition cannot be satisfied.

One way to meet this poling difficulty is to raise the polymer matrixconductivity. In Japanese Kokai No. 56-93383, July 28, 1981, apiezoelectric material is prepared by dispersing fine piezoelectricceramic powder in a high permittivity polymer matrix consisting of aninsulating polymer and an organic substance with high conductivity.

SUMMARY OF THE INVENTION

In accordance with the present invention, a piezoelectric-polymercomposite of 0-3 connectivity having improved poling field strength isfabricated by increasing the electrical conductivity of the polymerphase. In a particular method, a third phase of metallic orsemiconductive material in particulate form is added to the polymermatrix to enhance the polymer matrix conductivity. As a result, theconductivity of the polymer matrix will increase to about 10⁻⁶ ohm⁻¹cm⁻¹ and that of the composite as a whole will increase to 10⁻¹⁰ ohm⁻¹cm⁻¹.

Increasing the conductivity of the polymer phase in the PZT-polymer 0-3composite greatly improves the poling conditions and consequently,inexpensive and useful 0-3 piezoceramic-polymer composite can befabricated.

BRIEF DESCRIPTIONS OF THE DRAWING

FIG. 1 is a flow chart of the process steps for forming the 0-3piezoelectric-polymer composites of the Example.

FIG. 2 is a graph of d₃₃ as a function of the poling field found duringtesting of piezoelectric composite samples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with the present invention, a piezoelectricceramic-polymer composite of 0-3 connectivity is prepared by blending apiezoelectric ceramic powder and a particulate additive havingconductive or semiconductive properties as a third phase into aninsulating polymer matrix.

The conductive or semiconductive phase may comprise an inorganic ororganic substance in particulate form. Among the various particulateadditives which can be utilized include carbon, germanium, silicon, andmetals such as silver, etc. Organic compounds may includetetracyanoquinodimethane (TCNQ). Generally, the conductive orsemiconductive third phase should have a conductivity of at least 10⁻³ohm⁻¹ cm⁻¹.

Any polymeric material with high insulating property able to be blendedwith the piezoelectric ceramic powder and third phase conductive orsemiconductive additive and capable of forming thin films can be usedbut the selection should be carefully considered according to theperformance demanded.

It is preferable for the polymer matrix to have as low as dielectricloss as possible. The examples of such polymers include polystyrene,polysulfone, polycarbonate, polypropylene, polyethylene, polyethyleneterephthalate, polyphenylene oxide, polyurethane elastomer, polyvinylcarbazole and polyvinylidene fluoride. The mixtures of theaforementioned polymers and the copolymers of the aforementionedpolymers could also be used. On the other hand if a low dielectric lossis not required any polymer could be used and the selection can be basedon the ability to form thin films, heat resistance, mechanicalproperties, electrical properties and strength. For example,polyacrylate esters, polymethacrylate esters, polyvinyl chloride,polyvinylidenes, polymers of acrylonitrile, polymers ofmethacrylonitriles, polyvinyl acetate, polyvinyl pyrrolidone, cellulosetype polymers such as ethyl cellulose, soluble polymers of polyimide,epoxy resins, curable polymers such as unsaturated polyester resins canbe used. The mixtures of the aforementioned polymers and the copolymersof aforementioned polymers could be used.

It is preferable to blend less than 4 vol. % of the conducting orsemiconducting third phase particulate into the polymer matrix. Theamount of the third phase additive will depend upon the conductivitythereof. Thus, depending upon the conductivity of the third phaseadditive, about 1.5 to about 4 vol. % will be incorporated into thepolymer matrix.

The piezoelectric phase can be selected from among any of thepiezoelectric ceramic powders including quartz, barium titanate, leadtitanate, lead niobate, PZT, etc,. The amount of fine piezoelectricceramic powder used should be preferably selected so that the level ofpiezoelectric ceramic content is about 60 to about 70 vol. % of thepiezoelectric ceramic-polymer composite. Although the piezoelectricratio of the 0-3 composite obtained increases with the increasing amountof piezoelectric powder used, the mechanical properties and operationproperties are adversely affected by the increasing level ofpiezoelectric ceramic phase. Thus, it is preferably to select the amountof piezoelectric ceramic powder to the range specified above. Table 1illustrates piezoelectric materials which are useful in this invention.

                  TABLE 1                                                         ______________________________________                                        (1) Single component;                                                         BaTiO.sub.3, PbTiO.sub.3, PbNb.sub.2 O.sub.6,                                 (Na.sub.1/2 Bi.sub.1/2)TiO.sub. 3, LiNbO.sub.3, LiTaO.sub.3                   (2) Two component;                                                            PbTiO.sub.3 --PbZrO.sub.3,                                                    PbTiO.sub.3 --Pb(Zn.sub. 1/3 Nb.sub.2/3)O.sub.3,                              PbTiO.sub.3 -- Pb(Cd.sub.1/2 W.sub.1/2)O.sub.3,                               PbTiO.sub.3 -- Pb(In.sub.1/2 Nb.sub.1/2)O.sub.3,                              KNbO.sub.3 --NaNbO.sub.3,                                                     Cd.sub.2 Nb.sub.2 O.sub.7 --NaNbO.sub.3,                                      PbNb.sub.2 O.sub.6 --BaNb.sub.2 O.sub.6                                       (3) Three component;                                                          PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3,                                       --Pb(Co.sub.1/3 Nb.sub.2/3)O.sub.3,                     PbTiO.sub.3 --PbZrO.sub.3 --Pb(Zn.sub.1/3 Nb.sub.2/3)O.sub.3,                                       --Pb(Cd.sub.1/3 Nb.sub.2/3)O.sub.3,                     PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mn.sub.1/3 Nb.sub.2/3)O.sub.3,                                       --Pb(Ni.sub.1/3 Nb.sub.2/3)O.sub.3,                     PbTiO.sub. 3 --PbZrO.sub.3 --Pb(Sb.sub.1/2 Nb.sub.1/2)O.sub.3,                                      --Pb(Y.sub.1/2 Nb.sub.1/2)O.sub.3,                      PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mg.sub.1/2 W.sub.1/2)O.sub.3,                  PbTiO.sub.3 --PbZrO.sub.3 --Pb(Co.sub.1/2 W.sub.1/2)O.sub.3,                                        --Pb(Cd.sub.1/2 W.sub.1/2)O.sub.3,                      PbTiO.sub.3 --PbZrO.sub.3 --Pb(Mn.sub.1/2 Te.sub.1/2)O.sub.3,                                       --Pb(Mn.sub.1/3 Sb.sub.2/3)O.sub.3,                     PbTiO.sub.3 --PbZrO.sub.3 --Pb(Ni.sub.1/3 Sb.sub.2/2)O.sub.3,                                       --Pb(Fe.sub.1/2 Nb.sub.1/2)O.sub.3,                     Others                                                                        PbTiO.sub.3 --Ba(Zn.sub.1/3 Nb.sub.2/3)O.sub.3 --Pb(Zn.sub.1/3 Nb.sub.2/3)    O.sub.3,                                                                      KNbO.sub.3 --NaNbO.sub.3 --LiNbO.sub.3                                        (Pb.sub.1-x La.sub.x)(Zr.sub.y Ti.sub.2).sub.1-x /.sub.4 O.sub.3                                    (x + y = 1)                                             ______________________________________                                    

The piezoelectric ceramic-polymer composite of this invention isprepared by the following procedure using the starting materialsdescribed above. In particular, the piezoelectric ceramic powder such asPZT is mixed with the particulate conductive or semiconductive thirdphase. The mixing is done such as by ball milling for about 2 hours. Thepolymer matrix is then added and mixed with the particulate phases andmolded into shape. If the polymer matrix is a thermosetting resin, themolded composite can be cured at elevated temperatures.

EXAMPLE 1

FIG. 1 shows the sample preparation procedure used in this example.Commercial PZT-501 powder was obtained from Ultrasonic Powder Inc.,South Plainfield, N.J., Eccogel 1365-0 (an eopxy formulation) wasobtained from Emerson and Cuming, W. R. Grace and Co., Canton, Mass.,and fine-grained carbon were used as piezoelectric filler, matrix, andadditive to make 0-3 piezoelectric-ceramic composites. The PZT was mixedwith the carbon powder and ball milled for 2 hours. The polymer matrixwas then added to the particulate phase and mixed therein. The PZTcompound comprised 68.5 vol. % of the PZt polymer composite. Sampleswere cut 1 cm×1 cm×0.1 cm. in dimension and an air-dry silver electrodewas painted on both surfaces for poling and other transducer propertiesinvestigations.

The hydrostatic voltage coefficient g_(h) is about 6 to 10 times largerthan that of single phase PZ. Similarly, the FIG. of Merit d_(h) g_(h)is 11/2 to 8 times larger than that of single phase PZT. The results aresummarized in Table 2.

                                      TABLE 2                                     __________________________________________________________________________    Comparison of Dielectric and Piezoelectric Properties of                      PZT-Polymer Composites (with and without Conductive Third Phase)                         Vol. % of       - d.sub.33                                                                             - g.sub.h                                                                              - d.sub.h                                                                              - g.sub.n -                                                                   d.sub.h                            Third Phase                                                                           tan δ                                                                       K.sub.33                                                                          (× -12 C/N)                                                                      (× -3 Vm/N)                                                                      (× -12                                                                           (× -15                                                                  m.sup.2 /N)             __________________________________________________________________________    Solid PZT  --      0.015                                                                             1800                                                                              450      2.5      40       100                     PZT-Polymer                                                                              0       0.03                                                                              100 45       9        10        90                     Composite                                                                     PZT-Carbon-Eccogel                                                                       1.5     0.08                                                                              120 50       30       30       900                     Composite                                                                     PZT-Ge-Eccogel                                                                           4       0.08                                                                               90 44       17       22       375                     Composite                                                                     PZT-Si-Eccogel                                                                           1.5     0.075                                                                              85 45       18       23       415                     Composite                                                                     __________________________________________________________________________     Polymer: Eccogel 13650                                                        Volume Percent of Polymer is 30                                          

EXAMPLE 2

FIG. 2 shows d₃₃ as a function of poling conditions. By optimizing d₃₃coefficient, the poling behavior of carbon added PZT-Eccogel 0-3composites were studied. PZT-Eccogel 0-3 samples with 1.5 volume percentcarbon additive can be poles sufficiently in a 1209° C. oil bath, at 35kV/cm, for about 5 minutes. This result is much improved poling behaviorcompared to 0-3 PZT-Eccogel composite without carbon additive. Theoptimized d₃₃ values varied between 48 to 50×10¹² C/N as shown in FIG.2.

What is claimed is:
 1. A transducer comprising a poledpiezoelectric-polymer composite material of 0-3 connectivity and whichcomprises: a matrix of insulating polymer, a piezoelectric ceramicpowder dispersed within said polymer matrix and an inorganic conductiveor semiconductive particulate additive dispersed within said polymermatrix, said inorganic additive having an electrical conductivitygreater than the electrical conductivity of said insulating polymermatrix.
 2. The transducer of claim 1 wherein the said particulateadditive in said composite comprises about 1.5 to about 4 volume % ofsaid composite.
 3. The transducer of claim 1 wherein said particulateadditive has an electrical conductivity of at least about 10⁻³ ohm⁻¹cm⁻¹.
 4. The transducer of claim 1 wherein said particulate additivecomprises is a conductive metal.
 5. The transducer of claim 1 whereinsaid particulate additive is a semiconductive material.
 6. Thetransducer of claim 5 wherein said semiconductive material is carbon. 7.The transducer of claim 5 wherein said semiconductive material isgermanium.
 8. The transducer of claim 5 wherein said semiconductivematerial is silicon.
 9. The transducer of claim 1 wherein saidpiezoelectric ceramic powder comprises at least about 60 vol. % of saidceramic powder and polymer mixture.
 10. The transducer of claim 1wherein said insulating polymer is an epoxy resin.
 11. The transducer ofclaim 1 wherein said piezoelectric ceramic powder is lead zirconatetitanate.